I’d actually suppose this is a bug but I cannot imagine that no one should have fixed this by now:
In KDE System settings > Application Appearance you can select colouring for various elements of the desktop. So far so good.
I’ve changed the colour of the ‘Active Title Bar’ to some darker blue. After a reboot of the system this change is undone: My title bars have the same old bright blue as before.
Can anyone confirm this experience? Is this a bug?